Lots of you explained your reasoning, and had worked very
systematically. One person made a guess at one point, which turned
out to be right. However there is sometimes more than one solution
to this sort of problem, so it's always worth following up the
other possibility to check! Congratulations are due to Freddie
Manners, who noticed that in fact you did not need to know what A
and D were.

The first and fourth columns must add up to different numbers.
There can't be a carry from the second column (A is 3), so there
must be a carry from the fifth column to the fourth, so H+H must be
at least 10. This means L will be odd.

C must be less than 5 (looking at the first column). It can't be
2 or 3, and it can't be 1, since that would make E be 2. So C is 4,
L is 9, E is 8, and the third column tells us that U is 7, since it
can't be 2.

We now have:

4

3

7

4

H

Y

4

3

7

4

H

Y

-

-

-

-

-

-

8

7

4

9

I

2

The only numbers are left are 1, 5 and 6, and we know that Y is
1 or 6. It is quick to try these and see that the only solution is
when Y is 6, H is 5 and I is 1. Therefore CAYLEY is 436986.

Several people had found out about the three mathematicians.
Here is a brief summary: if you click on the names, you can read
the biographies on the St Andrews
University History of Maths website.
Cayley was a 19th century British mathematician, who worked on
matrices, and geometry in more than 3 dimensions.
Cauchy was a Frenchman, and Michael Brooker claims that he is
best known for having far more theorems named after him than any
other mathematician!
Euclid was a Greek, born around 325 BC, who worked on
geometry.

The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the
NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to
embed rich mathematical tasks into everyday classroom practice. More information on many of our other activities
can be found here.